1. Field of the invention
The present invention relates to a road surface state estimating device and, more particularly, to a road surface state estimating device (a device for estimating the state of a road surface) that estimates a physical quantity representing a road surface state such as a braking force gradient and a driving force gradient in a low slip region including a steady traveling (running) region.
2. Description of the Related Art
Japanese Patent Application Laid-Open (JP-A) No. 2000-118375 discloses an ABS device for estimating a braking torque gradient (obtained by a braking force gradient being multiplied with a square of a wheel effective radius) from a wheel speed signal, and maximizing braking force by controlling the estimated braking torque gradient to coincide with a target value near zero. In this device, the braking torque gradient is estimated on the basis of a wheel deceleration motion model represented by the following Equation (1), whereby the braking torque gradient, therefore, the braking force gradient can be accurately estimated in a limit braking region where a wheel deceleration motion is dominant.                                           ν            ¨                    w                =                                            -                                                kR                  c                  2                                J                                      ⁢                                          ν                .                            w                                +          w                                    (        1        )            
where xcexdw represents a wheel speed (m/s); w represents a road surface disturbance; k represents a braking force gradient (Ns/m); RC represents an effective radius of the tire (m); and J represents moment of inertia of a vehicle.
However, in a low slip region where the braking force gradient is relatively large, the wheel deceleration motion is affected by a suspension longitudinal resonance, which is a resonance generated at near 15 Hz, and a tire rotation resonance, which is a resonance generated at near 40 Hz. Therefore, there is a problem in that it is not possible to accurately estimate the braking force gradient in the low slip region by the technique in which the braking torque gradient is estimated on the basis of the Equation (1).
Japanese Patent Application Laid-Open (JP-A) No. 11-78843 discloses a wheel state estimating device, wherein a braking force gradient is estimated on the basis of a tire rotation vibration model. By noting that resonance characteristics of the tire rotation vibration become sharper as the braking force gradient becomes larger, a damping coefficient of the tire rotation vibration is identified to estimate the braking force gradient.
However, in a case of braking in which the braking force gradient becomes small, because wheel deceleration motion becomes dominant, tire rotation vibration is not generated. Therefore, in the prior art described above, there is a problem in that the braking force gradient cannot be estimated in a case of braking in which tire rotation vibration is not generated.
The present invention has been conceived to solve the above problem. It is an object of the present invention to provide a road surface state estimating device which estimates a physical quantity representing a road surface state such as a braking force gradient, a driving force gradient and a road surface xcexc gradient in a low slip region including a steady travelling (running) region.
Principles of the present invention will now be described. As shown in FIG. 1, a wheel resonance system can be represented by a dynamic model in which torsional spring elements 14 and 16 of a tire, having respective spring constants K1 and K2, are interposed between a rim 10 and a belt 12 and in which a suspension element, in which a spring element 18 having a spring constant K3 is connected in parallel with a damper 20, is interposed between the rim 10 and a vehicle body. In this model, a disturbance from the road surface (road surface disturbance) is transmitted from the belt 12 through the spring elements 14 and 16 to the rim 10, to affect a wheel speed xcfx89, and is transmitted to the vehicle body through the suspension element.
Description will now be given of relation between the braking force gradient and a wheel speed frequency characteristic quantity representing following frequency of transmission characteristics from a road surface disturbance to the wheel speed, using a fifth order full wheel model, in which a first order wheel decelerating motion, second order longitudinal direction suspension resonance, and second order tire rotation resonance are integrated. The braking force gradient is represented, as shown in FIG. 2, by a gradient of a tangent of a curve representing a relationship between a braking force and a slip speed (or slip rate).
FIG. 3 is a gain diagram showing frequency responses from a road surface disturbance to the wheel speed for ranges from a limit braking range to a low slip range where there is some margin for tire characteristics (i.e., for ranges from a range at which the braking force gradient is 300 Ns/m to a range at which the braking force gradient is 10000 Ns/m). That is, the diagram shows the relationship between frequency and gain of amplitude of the wheel speed with respect to amplitude of the road surface disturbance.
The wheel speed frequency characteristics in FIG. 3 indicate that, for the range where the braking force gradient is relatively small, such as near the limit of friction force between a tire and a road, the gain is large in a low frequency range and is small in a high frequency range. Namely, for the range where the braking force gradient is relatively small, the wheel speed frequency characteristic quantity, which represents a difference between the gain in the low frequency range and the gain in the high frequency range, is large.
In contrast, the gain in the low frequency range for the range where the braking force gradient is relatively large, such as a steady traveling region, is much smaller compared to that for the range where the braking force gradient is relatively small, in the wheel speed frequency characteristics. Further, in the high frequency range, the gain for the range where the braking force gradient is relatively large is not much smaller than the gain for the range where the braking force gradient is relatively small due to the influence of rotational resonance of the tire (near 40 Hz) being generated. Therefore, for the range where the braking force gradient is relatively large, the wheel speed frequency characteristic quantity is small. Also, a wheel speed frequency characteristic quantity, which represents a difference between a vibration level of a wheel speed signal in the low frequency range and a vibration level of a wheel speed signal in the high frequency range, changes similarly to the wheel speed frequency characteristic quantity which represents the difference between the low frequency range gain and the high frequency range gain.
It is apparent from the above that the wheel speed frequency characteristic quantity which represents the difference between the low frequency range gain and the high frequency range gain, or the wheel speed frequency characteristic quantity which represents the difference between the wheel speed signal vibration level in the low frequency range and the wheel speed signal vibration level in the high frequency range, decreases as the braking force gradient increases. Utilizing this characteristic, the braking force gradient can be estimated from the wheel speed frequency characteristic quantity.
Referring to the frequency band near 40 Hz in FIG. 3 at which rotational resonance of the tire occurs, the greater the braking force gradient, the sharper the peak waveform of rotational resonance of the tire. Further, as the braking force gradient becomes greater, the overall frequency characteristics of the peak waveform move to a higher frequency range.
Namely, if the wheel characteristics is approximated to a first-order lag model, it can be understood that a break point frequency becomes higher as the braking force gradient becomes greater, as shown in FIG. 6. It is therefore possible to estimate the braking force gradient from the wheel speed frequency characteristic quantity which represents the following frequency of transmission characteristics from the road disturbance to the wheel speed, by approximating the wheel characteristics to the first-order lag model and estimating the break point frequency (as the wheel speed frequency characteristic quantity), which is a frequency at which the gain changes from a value in a predetermined range to a value out of the predetermined range. Lag models of the second and third orders and the like have characteristics substantially similar to those of the first-order lag model. Therefore, it is possible to estimate a braking force gradient from the value of the wheel speed frequency characteristic quantity, by approximating wheel characteristics to the lower order lag model and estimating the wheel speed frequency characteristic quantity.
The braking force gradient when a braking force is applied to the tire and a driving force gradient when a driving force is applied to the tire are both physical quantities representing slipperiness between the tire and the road surface. Also, the gradient of braking force and the gradient of driving force are physical quantities equivalent to the xcexc-gradient of the road surface, which represents tire grip state. Accordingly, any one of the braking force gradient, which is a gradient of a tangent of a curve that represents a relationship between a slip rate (or, a slip speed) and a braking force, the driving force gradient, which is a gradient of a tangent of a curve that represents a relationship between a slip rate (or, a slip speed) and a driving force, and a xcexc-gradient of the road surface, which is a gradient of a tangent of a curve that represents a relationship between a slip rate (or, a slip speed) and a road surface xcexc, can be estimated as a physical quantity representing slipperiness between the tire and the road surface.
The present invention has been conceived based on the above-described principle. A first aspect of the present invention comprises: a wheel speed sensor for detecting a wheel speed and outputting a wheel speed signal; a wheel speed frequency characteristic quantity estimating section for estimating a wheel speed frequency characteristic quantity which represents a following frequency of a transmission characteristics from a road disturbance to the wheel speed on the basis of the wheel speed signal; and a physical quantity estimating section for estimating a physical quantity which represents a road state, from the estimated wheel speed frequency characteristic quantity.
A second aspect of the present invention comprises: a wheel speed sensor for detecting a wheel speed and outputting a wheel speed signal; a wheel speed frequency characteristic quantity estimating section for estimating a wheel speed frequency characteristic quantity which represents a difference between a characteristic quantity in a low frequency region and a characteristic quantity in a high frequency region which is higher than the low frequency region in a gain diagram representing a frequency response of a transmission characteristics from a road surface disturbance to the wheel speed on the basis of the wheel speed signal; and a physical quantity estimating section for estimating a physical quantity which represents a road state, from the estimated wheel speed frequency characteristic quantity.
A third aspect of the present invention comprises: a wheel speed sensor for detecting a wheel speed and outputting a wheel speed signal; a wheel speed frequency characteristic quantity estimating section for estimating a wheel speed frequency characteristic quantity which represents a following frequency of a transmission characteristics from a road disturbance to the wheel speed on the basis of the wheel speed signal; and a physical quantity estimating section for estimating a physical quantity which represents a road state, from the estimated wheel speed frequency characteristic quantity. In the third aspect of the present invention, the wheel speed frequency characteristic quantity represents, in a transmission characteristics which is from a road surface disturbance to the wheel speed approximated to a low order model, a break point frequency which is a frequency, at which gain changes from a value in a predetermined range to a value out of the predetermined range, in a gain diagram representing a frequency response of the approximated low order model, on the basis of the wheel speed signal.
A fourth aspect of the present invention comprises: a wheel speed sensor for detecting a wheel speed and outputting a wheel speed signal; a wheel speed frequency characteristic quantity estimating section for estimating a physical quantity as a wheel speed frequency characteristic quantity, for each of a plurality of frequency regions which are divided in a gain diagram representing a frequency response of a transmission characteristics from a road surface disturbance to the wheel speed on the basis of the wheel speed signal; and a physical quantity estimating section for estimating a physical quantity which represents a road state, from the plurality of the estimated wheel speed frequency characteristic quantities.
The gain diagram is a diagram which represents the relationship between a frequency and a gain represented by a ratio of an amplitude of an output (an amplitude of time series data of a wheel speed) with respect to an amplitude of an input (an amplitude of road surface disturbance). A sensor that detects wheel speed in a predetermined sampling period and outputs time series data of the wheel speed can be used as the wheel speed sensor.
In the third aspect of the present invention, as the low order model, a first order lag model, a first order lag model or the like can be used, but the first order lag model is preferable.
In the each of the aspects of the present invention, taking advantage of the fact that overall frequency characteristics (waveform) in a frequency band near 40 Hz, in which a tire rotation resonance is generated, move to a higher frequency as the braking force gradient becomes larger, a physical quantity representing slipperiness between the tire and the road surface is estimated as a physical quantity representing the road surface state. Accordingly, a physical quantity representing the state of the road surface in a low slip region including a steady travelling (running) region can be estimated.